Conventionally, radiography used for medical image diagnosis has been classified into general radiography such as X-ray radiography for obtaining a static image, and fluoroscopic radiography for obtaining a moving image. Each radiography including a imaging apparatus and an image pickup apparatus are selected as occasion demands.
In the conventional general radiography, two systems described below have mainly been implemented. One is a screen film imaging (abbreviated as SF imaging hereinafter) system which executes imaging through film exposure, developing, and fixing by using a screen film prepared by combining a fluorescent screen and a film. The other is a computed radiography imaging (abbreviated as CR imaging hereinafter) system which records a radioactive ray image as a latent image in an photostimulable phosphor, scans the accelerated phosphorescence fluorescent material with a laser to output optical information according to the latent image, and reads the output optical information by a sensor. However, in the conventional general radiography, there has been a problem in that a process of obtaining the radioactive ray image is complex. The obtained radioactive ray image can be converted into digital data, but it is indirectly digitized. Thus, there arises a problem in that it takes much time to obtain digitized radioactive ray image data.
Next, in the conventional fluoroscopic radiography, an image intensifier radiography (abbreviated as I. I. radiography hereinafter) system that uses a fluorescent material and an electron tube has mainly been employed. However, in the conventional fluoroscopic radiography, there has been a problem in that an apparatus is large-scaled because of use of the electron tube. The use of the electron tube has created difficulty in obtaining an image having a large area because of a small field of view (detection area). Furthermore, there has been a problem in that a resolution of an obtained image is low because of the use of the electron tube.
Thus, in recent years, attention has been focused on a sensor panel configured by arranging, in a 2-dimensional matrix, a plurality of pixels having conversion elements and switch elements for converting radioactive rays or lights from fluorescent materials into charges. In particular, attention has been focused on a flat panel detector (abbreviated as FPD hereinafter) in which a plurality of pixels having conversion elements prepared by, on an insulating substrate, non-single crystal semiconductors such as amorphous silicon (abbreviated as a-Si hereinafter) or the like, and thin-film transistors are arranged (abbreviated as TFT hereinafter) prepared by non-single crystal semiconductors in a 2-dimensional matrix.
This FPD can obtain an electric signal based on the image information by converting, by the conversion element, radioactive rays having image information into charges, and reading the charges by the switch element. Accordingly, the image information can be directly taken out as digital signal information from the FPD, so handling of image data such as storage, processing, and transfer is facilitated to enable further use of the radioactive ray image information. Characteristics such as sensitivities of the FPD depend on radiography conditions. As compared with the conventional SF or CR imaging system, however, equal or better characteristics have been verified. Additionally, as the electric signal having the image information can be directly obtained from the FPD, as compared with the conventional SF or CR imaging system, there is an advantage that time necessary for obtaining an image is shortened.
As the FPD, as described in International Application Publication No. WO93/14418, a PIN type FPD has been known which uses a sensor panel configured by arranging, in a 2-dimensional matrix, a plurality of pixels formed of PIN type photodiodes made of a-Si and TFTs. Such the PIN type FPD has a laminated structure in which a layer constituting a PIN type photodiode is disposed on a layer constituting the TFT on a substrate. As described in U.S. Pat. No. 6,075,256, an MIS type FPD has been known which uses a sensor panel configured by arranging, in a 2-dimensional matrix, a plurality of pixels formed of MIS type photosensors made of a-Si and TFTs. Such the MIS type FPD has a plane structure in which the MIS type photosensor is disposed by the same layer configuration as that constituting the TFT on the substrate. Furthermore, as described in U.S. Application Publication No. US-2003-0226974, an MIS type FPD of a laminated structure has been known in which a layer constituting the MIS type photosensor is disposed on the layer constituting the TFT on the substrate.
The above-mentioned FPD will be described below by taking the example of U.S. Application Publication No. US-2003-0226974 with reference to the drawings. For simpler explanation, an example of an FPD arranged at a 3×3 2-dimensional matrix will be taken.
FIG. 6 is a schematic equivalent circuit diagram showing an equivalent circuit of a conventional FPD described in U.S. Application Publication No. US-2003-0226974. FIG. 7 is a schematic plan diagram of one pixel of the conventional FPD described in U.S. Application Publication No. US-2003-0226974. FIG. 8 is a schematic sectional diagram cut on the line 8-8 of FIG. 7.
By the FPD configured as described above, light emitted from a wavelength converter according to incident radioactive rays are converted into signal charges by a plurality of photoelectric conversion elements to which photoelectric conversion biases have been applied. A plurality of switching elements perform transfer operations according to a drive signal applied to the drive wiring by a drive circuit, whereby the signal charges converted by the photoelectric conversion elements are transmitted through a signal wiring to be read in parallel to a signal processing circuit. The signal charges read in parallel are converted into serial signals by the signal processing circuit, and converted from analog signals into digital signals by an A/D converter to be output. Through the above operation, it is possible to obtain an image signal of one pixel according to the incident radioactive rays containing image information.
In intersection portions of the FPD having the above laminated structure, the wirings are insulated from each other via the insulating layers. However, as reliabilities at the intersection portions are greatly affected by manufacturing yield or image quality, insulating property is highly required among the wirings. In particular, signal charges generated by the photoelectric conversion element and transferred by the switching element flow to the signal wiring. Thus, leakage between the signal wiring and another wiring totally reduces a quality of the FPD. Further, the influence of parasitic capacitance or wiring resistance of the signal wiring causes noise of an image signal to be output, creating a possibility of adversely affecting the image signal. In particular, the radiation detecting apparatus which outputs signal charges by a small exposure radiation dosage and must have a high sensitivity, as a noise influence is large because of small signal charges generated by the photoelectric conversion element, it is necessary to reduce the influence of parasitic capacitance or wiring resistance of the signal wiring. Consequently, it is necessary to secure insulation at the intersection portion of the signal wiring and the drive wiring, and insulation at the intersection portion of the signal wiring and a bias wiring, which cause parasitic capacitance in the signal wiring. It is particularly necessary to secure insulation between the signal wiring and the bias wiring. In addition, it is required to reduce the parasitic capacitance or wiring resistance. Reasons for this will be described below.
As described above, a first insulating layer, a first semiconductor layer, and a first impurity semiconductor layer similar to those used for the switching element are present between the wirings at the intersection portion of the signal wiring and the drive wiring at the intersection portion. As these layers are formed in the process of forming the switching element, layer quality is high, and the first insulating layer exhibits very high insulating property because it is used for the gate insulating film of the switching element. Thus, as the insulating property is high between the signal wiring and the drive wiring at the intersection portion, the drive wiring is formed thick and the wiring width is narrowed so as to reduce the influence of parasitic capacitance, whereby the influence of noise can also be reduced. On the other hand, an interlayer insulating layer, a second insulating layer, a second semiconductor layer, and a second impurity semiconductor layer are present between the signal wiring and the bias wiring at the intersection portion. As these layers are formed after the switching element is formed, a forming temperature thereof must be lower than an endurance temperature of the switching element. Generally, since the endurance temperature of the switching element is lower than the forming temperature, the interlayer insulating layer formed in an upper layer thereof is formed at a temperature lower than that of the first insulating layer. Even when an inorganic material similar to that of the first insulating layer is used for the interlayer insulating layer, its insulation is low due to the low forming temperature. Even when an organic material also serving as a planarized film is used, although different from the case of the first insulating layer, to form the interlayer insulating film its insulating property is lowered as the organic material has lower insulating property compared to that of the inorganic material in most cases. When the MIS type photosensor is used, the second insulating layer that can be made of the same material as that of the first insulating layer is disposed, however, the second insulating layer is formed at a temperature lower than that of the first insulating layer as in the case of the interlayer insulating layer. Accordingly, the second insulating layer has lower insulating property than the first insulating layer. As described above, the insulating property at the intersection portion between the signal wiring and the bias wiring is lower than that at the intersection portion between the signal wiring and the gate drive wiring.
On the other hand, a reduction in wiring resistance which causes noise in the signal wiring is required. For the wiring resistance, generally, the wiring is formed thick or large in width. However, in the FPD configured by arranging the wirings in the matrix, when the wiring is formed large in width, an area at the intersection portion between the wirings becomes large which causes an increase in parasitic capacitance. Thus, the wiring is not formed so large in width. Hence, the wiring resistance is reduced mainly by forming the wiring thick.
However, when the signal wiring is formed thick, it is accompanied by an enlargement of a step and it is difficult to perform microprocessing, which makes it difficult to control a processing form. When the step is enlarged by the signal wiring and the processing form is deteriorated, it is difficult to uniformly form the interlayer insulating film disposed by covering the signal wiring. When an inorganic material is used for the interlayer insulating film, it is difficult to form the interlayer insulating film to be thick. Thus, it is difficult to form the interlayer insulating film which covers a side surface of the signal wiring to be in similar thickness to that of the surface. Accordingly, at the intersection portion between the signal wiring and the bias wiring, insulating property is reduced between the side surface of the signal wiring and the bias wiring, thus increasing possibilities of leakage and of generation of uneven line images. In other words, when each wiring is formed thick for the purpose of reducing noise, leakage occurs between the wirings. Moreover, prevention of leakage leads to insufficient improvement in terms of noise.